Implications from Thermoelastic Stresses on the Lifecycle of Injection and Production Wells in Enhanced Geothermal Systems

ABSTRACT: Geothermal well integrity, a subdiscipline that evolved from conventional oil-and-gas well integrity concepts, often requires special considerations. Temperature variations (gradients) typically found in around geothermal wells engender the need for incorporating induced thermoelastic stresses for any stress-deformation analyses for the near-wellbore vicinity. Such thermoelastic stresses can be described mathematically using closed-form expressions. Equations for stress-deformation behaviors then can be developed incorporating these thermoelastic stresses. In this paper, the impacts from thermal effects on the stress distributions within a geothermal well's casing-cement sheath-rock formation (C/CS/RF) system are assessed quantitatively for two well types used in an enhanced geothermal system (EGS): an injection well undergoing borehole cooling and a production well undergoing borehole heating. A novel method is used for the initial stress distributions within the intermediate-CS layers, calibrating for "free strains" created during the wait-on-cement (WOC) period, for a stable initial (post-WOC) state. Borehole cooling on the EGS injection well promotes casing collapse, but suppresses radial cracking within the intermediate-CS layer and longitudinal-fracture initiations (as opposed to ideally-oriented transverse) from the CS/RF interface. Contrary, borehole heating on the EGS production well promotes burst failures within the casing walls, radial cracks within the intermediate-CS layer, and longitudinal-fracture initiations from the CS/RF interface.